A neural substrate underlying odor-evoked locomotion was revealed in lampreys (Derjean et al., 2010), involving a neural pathway extending from the medial part of the olfactory bulb to the posterior tuberculum. The signal is then relayed to the mesencephalic locomotor region and eventually reaches reticulospinal cells that activate the spinal locomotor networks. Recent research in the lab (Daghfous et al., 2013) shows that the medial olfactory bulb, is under a tonic GABAergic inhibition gating reticulospinal cell responses to olfactory nerve stimulation. Additional modulatory mechanisms might exist in the olfactory bulb of Petromyzon marinus as it also contains dopaminergic fibers. In every vertebrate studied to date, dopamine plays an important role in olfactory processing.
Anatomical (axonal tracers and immunofluorescence) and physiological (intracellular recordings) techniques were used to investigate the dopaminergic modulation of the olfacto-motor pathway. Dopamine immunofluorescence showed scarce innervation of the olfactory bulb that was most abundant in the medial part and in close vicinity to projection neurons and olfactory nerve fibers. Additionally, no dopamine-immunoreactive cell bodies were detected in the olfactory bulb. Synaptic responses of reticulospinal cells to olfactory nerve stimulation were recorded in the isolated brain. Local injection of dopamine in the medial olfactory bulb induces an almost two-fold decrease of the synaptic responses. When GABAA receptor antagonist GABAzine was also injected in the medial olfactory bulb, the effect of dopamine was much more evident and could suppress large bursts of action potentials. However, D1 (SCH 23390) and D2 (Eticlopride) receptor antagonists injection in the medial olfactory bulb failed to alter the amplitude of reticulospinal cell responses to olfactory nerve stimulation, indicating that this modulation is not tonic. To locate the neurons responsible for this modulation, tracer injections combined with dopamine immunofluorescence were performed. Dopaminergic cells projecting to the medial olfactory bulb were found in the dopaminergic nuclei of the posterior tuberculum and the periventricular hypothalamus. Altogether, our results show anatomically and physiologically the presence of a dopaminergic innervation within the medial olfactory bulb that mediates inhibitory effects on olfacto-motor signaling.